Conventional display devices typically have two channels for transmitting signals with different characteristics.
As shown in FIG. 1 of one conventional connection arrangement of a display device, the display device 11 is coupled to a personal computer 13 and a workstation 15 at the same time via signal leads 131, 151 respectively. The personal computer 13 outputs DSB type signal 131 to the display device 11 and the workstation 15 outputs BNC type signal 151 to the display device 11. It is well known in the arts that the BNC type signals and the DSB type signals are transmitted via cables of 5 leads (pins) and 15 leads (pins) respectively. The personal computer 13 may be coupled to the workstation 15 through a network line 153. It is well known in the art that the DSB type signal 131 has a relative lower frequency as compared to a relative higher frequency the BNC type signal 151 has.
As shown in FIG. 2 of another conventional connection arrangement of a display device, a workstation 21 outputs the text information of lower frequency and the graphics information of higher frequency to the display device 23 via the DSB signal lead 211 and BNC signal lead 213 respectively.
In the configurations shown in FIG. 1 and FIG. 2, the display device, responsive to a switch command input from the front panel of the display device by the user, displays the information corresponding to the selected DSB or BNC type signals.
In the following elaborations of the conventional signal switching circuit or the instant invention, only the circuit corresponding to the RED gun of the display device is shown and recited for sake of simplicity. The disclosed circuit and corresponding recitation are also readily applied to GREEN and BLUE guns in a similar manner for persons of ordinary skill.
In one conventional signal switching circuit for DSB and BNC signals, as shown in FIG. 3, the microprocessor (not shown) within the display device outputs a DBBNC signal 35 to control the switching between the DSB and BNC signals. The RED output terminal 37 is formed at the terminal connecting the emitters of the analog switch transistors Q100 and Q102. When the DBBNC signal 35 is 0 volt (logic low), the transistor Q193 is cutoff and the transistor Q192 is saturated. The bases of the transistors Q100 said Q102 have voltage values around 0.8 volts. The cathode of the diode D1 has a voltage value higher than its anode and, accordingly, D1 is off. The base of Q100 has a voltage value of 1.8 volts, the emitter of Q100 has a voltage of 2.5 volts and, accordingly, Q100 is active. In another respect, the cathode of the diode D2 has a voltage value lower than its anode and, accordingly, D2 is conducting. The voltage potential of the base of the transistor Q102 is elevated to 3.5 volts and, accordingly, Q102 is cut-off. Under the aforesaid condition, the video signal input to the BNC signal lead 31 is output to the RED output lead 37 and utilized by downstream circuits in a well known manner.
On the contrary, as DBBNC signal 35 is 5 volts (logic high), the video signal input to the DSB signal lead 33 is output to the RED output lead 37 and utilized by downstream circuits in a well known manner.
By analyzing the circuit in FIG. 3, it is found, in unexpected situations where the DSB channel has input signals of higher frequency, cross-talk between the input signals 31, 33 occurs resulting from the appreciable effect of capacitance existing between the collector and base terminals of the analog switch transistor Q100 or Q102. As a result, when the information corresponding to the DSB type signal 33 is being displayed on the display device, the interference effect from the BNC source 31 accompanies the display information. And, vice versa, when the information corresponding to the BNC type signal 31 is being displayed on the display device, the interference effect from the DSB source 33 accompanies the display information.